Various alternatives to conventional DNA (deoxyribonucleic acid) sequencing have been proposed. One such alternative approach, utilizing an array of oligonucleotide probes synthesized by photolithographic techniques, is described in Pease, et al., “Light-Generated Oligonucleotide Arrays for Rapid DNA Sequence Analysis,” Proc. Natl. Acad. Sci. USA, Vol. 91, pp. 5022-5026, May 1994. In this approach, the surface of a solid support modified with photolabile protecting groups is illuminated through a photolithographic mask, yielding reactive hydroxyl groups in the illuminated regions. A 3′ activated deoxynucleoside, protected at the 5′ hydroxyl with a photolabile group, is then provided to the surface such that coupling occurs at sites that had been exposed to light. Following coupling and oxidation, the substrate is rinsed and the surface is illuminated through a second mask to expose additional hydroxyl groups for additional coupling. A second 5′ protected activated deoxynucleoside base is presented to the surface. The selective photodeprotection and coupling cycles are repeated to build up levels of bases until the desired set of probes is obtained. It may be possible to generate high density miniaturized arrays of oligonucleotide probes using such photolithographic techniques wherein the sequence of the oligonucleotide probe at each site in the array is known. These probes can then be used to search for complementary sequences on a target strand of DNA, with detection of the target that has hybridized to particular probes accomplished by the use of fluorescent markers coupled to the targets and inspection by an appropriate fluorescence scanning microscope. A variation of this process using polymeric semiconductor photoresists, which are selectively patterned by photolithographic techniques, rather than using photolabile 5′ protecting groups, is described in McGall, et al., “Light-Directed Synthesis of High-Density Oligonucleotide Arrays Using Semiconductor Photoresists,” Proc. Natl. Acad. Sci. USA, Vol. 93, pp. 13555-13560, November 1996, and G. H. McGall, et al., “The Efficiency of Light-Directed Synthesis of DNA Arrays on Glass Substrates,” Journal of the American Chemical Society 119, No. 22, 1997, pp. 5081-5090.
A disadvantage of both of these approaches is that four different lithographic masks are needed for each monomeric base, and the total number of different masks required are thus up to four times the length of the DNA probe sequences to be synthesized. The high cost of producing the many precision photolithographic masks that are required, and the multiple processing steps required for repositioning of the masks for every exposure, contribute to relatively high costs and lengthy processing times. Techniques have been developed for the creation of arrays of probe sequences, polypeptides, and other large chain molecules using patterning processes that do not require multiple masks. See U.S. Pat. No. 6,375,903, and published United States patent application publication Nos. 2003/0068633, 2003/0143132, 2003/0143550, 2003/0143724, 2003/0148502, 2004/0126757, and 2004/0132029, which are incorporated herein by reference. The synthesis of oligomers in the production of high density microarrays is typically carried out on flat glass substrates. The amount of a particular oligomer synthesized in this manner is in the range of femtomoles. For example, for a typical oligomer density of about 20 picomoles/cm2, a DNA microarray 1.5 cm×1.7 cm in size will yield approximately 50 picomoles total. If the microarray chip contains 250 different oligomer sequences, the amount of each particular oligomer will be about 200 femtomoles, and likely even less taking into account the chip areas lost to controls, etc. It would be highly desirable to increase the amount of each oligomer to the 1-100 picomoles range. However, increasing the active area of the flat microarray substrate surface to increase the amount of oligomers produced is generally not practical.
Standard phosphoramidite synthesizers generate a small number of oligomers in amounts of several nanomoles or greater for each oligomer, while microarray synthesizers can generate a large number of different oligomers in parallel but in the femtomole range for each oligomer. For purposes of gene synthesis, an adequate amount of each oligomer would be in the 1-1000 picomoles range, which is enough material to yield the concentration needed for synthesis and assembly. Thus, much of the output of conventional synthesizers is unnecessary, and potentially wasted, while DNA microarrays are able to produce many more oligomers at one time than standard synthesizers but at less than optimum amounts.